The study of OSP as reliable surface finish of BGA substrate

OSPs(organic solderability preservatives) have been used as surface finish for over 20 years and continues to taken the place of HASL (hot air solder leveling) or other surface finish to ensure the solderability of printed circuit board (PCB)(Wengenroth and Stafstrom). It is a transparent organic complex film to coat onto copper surface to keep from corrosion of copper surface. The advantages of OSP are stronger solder joint, substrate low price, simple process and environment friendly. For BGA substrate surface finish, electrolytic Ni/Au and electroless Ni/Au are popular in packaging industry. To reduce the cost and solder joint embrittlement, OSP is getting popular in package assembly industry as BGA substrate surface finish, especially in CSP application. In this work, solder spreading is used to determine the wettability of Sn/3.OAg/O.5Cu with 3 kinds of flux. Flux B and C has higher activity and acidity that affected the solder wettability significantly. The thickness of OSP after each packaging process is measured by UV spectrophotometer. OSP discoloration is inspected after multi-reflow, N₂ implement and packaging process. After packaging process, the ball shear test, IMC growth analysis and failure mode as aging conditions were implemented to evaluate the OSP performance. Finally, these OSP packages mounted onto PCB to evaluate solder joint reliability by board level drop test.